In carrying out the rearing of water grasses and the synthetic culturing of algae in a water tank, the carbon dioxide necessary for photosynthesis has to be supplied and dissolved in water. In general, carbon dioxide is supplied to a water tank via a carbon dioxide bomb installed outside the tank, and it is difficult in this method to supply carbon dioxide evenly at a slow flow rate of 10 ml/minute or less. In order to rear ornamental water grasses, a method is employed in which carbon dioxide is filled in a diffusion cylinder with the one end thereof open in the water, and gas-liquid contact at the open end causes carbon dioxide to be dissolved in the water. However, the above method has the defect that the initial feeding supplying and stopping speed can not be controlled.
In order to solve such problems, one of the present inventors developed an apparatus for supplying dissolved carbon dioxide in which water is electrolyzed with carbon used as an anode in a liquid to which dissolved carbon dioxide is supplied as a mechanism for automatically supplying carbon dioxide without relying on the supply of carbon dioxide by an external supply source, such as a bomb, and has already filed a patent application thereon (JP-A-6-154760). In this invention, a specific embodiment comprises a cathode member formed by a perforated pipe made of cylindrical stainless steel, a carbon rod anode inserted in the cathode member in a non-contacting relationship, and a terminal fixed on the cathode member at the upper part thereof. According to the above structure, since not only the carbon anode can be compactly integrated with the cathode but also dissolved carbon dioxide and gaseous hydrogen formed on the cathode is emitted to a liquid from holes formed in the cathode member, it becomes possible to dissolve carbon dioxide into a liquid safely and efficiently.
However, continuing an electrolytic operation at a fixed current using the carbon dioxide-generating electrode apparatus with the structure described above has gradually increased the voltage between both electrodes while a considerable amount of the carbon anode remains and has sometimes resulted in causing a situation such that the current eventually does not flow between the electrodes and electrolysis is stopped.
Investigations repeatedly made by the present inventors in order to solve the cause of this problem have resulted in clarifying that the problem originates in the phenomenon that since in the existing structure described above, a cylindrical carbon anode having the same diameter throughout the length of the rod is inserted in a cylindrical cathode pipe having the same inner diameter in a longitudinal direction in a parallel non-contacting relationship such that the distance between the electrodes becomes fixed, the consumption of the carbon anode particularly in the vicinity of the upper part thereof connected directly to the terminal, proceeds dominantly to locally increase the insulation degree, and the carbon anode at the lower part thereof remains unreacted.